What Is A Dead Zone? An Examination Of Its Causes And Impact

When eating your favorite meal, the ocean is the last thing on your mind. But do you know that what we eat can significantly impact our oceans? Well, agriculture is one of the biggest causes of dead zones, which damages our marine life and ecosystems. So, what is a dead zone? 

Read on as we look at what a dead zone is, its types, how dead zones occur, their impact, and how our oceans can recover from dead zones. 

Related Read: How Much of the Ocean Has Been Explored?

What is a dead zone in the ocean? 

A dead zone, or, in scientific terms, an oxygen minimum zone, refers to a section of the ocean characterized by severely low oxygen levels⁴. The existence and survival of marine life hinge heavily on the availability of an ample concentration of dissolved oxygen. This necessary oxygen is deficient in a dead zone, creating environments unfit for plant and aquatic life.

Unfortunately, the depletion of oxygen levels in the ocean leads to the death of marine life or forces them to flee, destabilizing our marine ecosystem. These dead zones aren't limited to oceans. They also appear in lakes, rivers, and ponds.

Though dead zones can form naturally, human actions, including agricultural practices and climate change impacts, contribute significantly to their  

Types of dead zones           

We have four main types of dead zones based on how long the hypoxia lasts. This could be for hours, days, weeks, months, and even years. 

Permanent dead zones 

A permanent dead zone typically occurs in deep water where oxygen levels remain low throughout the year. It stays below two milligrams of dissolved oxygen per liter of water. 

Temporary dead zones              

The temporary dead zone stays hypoxic for only a short period. Temporary dead zones could be a few hours or days. 

Seasonal dead zones                           

Seasonal dead zones occur every year. It appears when warmer temperatures and increased rainfall have flushed more nutrients into the waterways.  

Diel cycling hypoxia      

The Diel cycling hypoxia is a type of dead zone that occurs in the warmer months but at night and early morning hours. 

How do dead zones occur? 

So, how exactly do dead zones form? Dead zones in oceans occur through a process called eutrophication. This happens when excess nutrients enter into the oceans and waterways. These extra nutrients include nitrogen and phosphorus, which stimulate the growth of algal blooms. 

The excessive growth of the algal bloom covers the water's surface to create dead zones. Some of these algal blooms may contain harmful toxins, which we then classify as harmful algal blooms. These toxic algae grow due to a type of algae known as blue-green algae. 

The harmful algal bloom eventually dies, sinking to the bottom of the ocean floor using up dissolved oxygen from the water. The decomposition process consumes oxygen, depleting the remaining oxygen available in the water. This decomposition process may happen so fast that aquatic organisms cannot escape the low-oxygen zone, resulting in suffocation and, ultimately, death. 

Elevated nutrient levels and the growth of blue-green algae can also contaminate drinking water in communities², causing animal and human illness.

Causes of ocean dead zones 

Here are a few causes of dead zones in our oceans and waterways: 

Agricultural pollution 

Agriculture is the most significant cause of nutrient pollution, leading to the formation of dead zones. According to a study, 60% of bays and coastal waters around the United States are affected by nutrient pollution from land-based activities. 

Intensive farming practices, especially in animal agriculture, lead to excess nutrients, including nitrogen and phosphorus from animal manure and chemical fertilizers, flowing into our waterways and ending up in coastal regions. As bacteria munch on animal waste, it increases carbon dioxide, producing less oxygen and harming aquatic species. 

Nitrogen and phosphorus from fertilizers used to grow feed for farm animals are primarily responsible for the Gulf of Mexico dead zone, which is the second-largest dead zone in the world. 

Vehicular and industrial emissions 

Agriculture isn’t the only source of nitrogen. Cars and other vehicles also release significant amounts of nitrogen and phosphorus into the atmosphere. These vehicular gas emissions can enter coastal waters, distorting the health of our oceans to create dead zones. 

Factory farms and power plants that burn fossil fuels to generate electricity are sources of nitrogen and phosphorus gases, which can penetrate our waters through the atmosphere. 

Climate change 

Temperature changes, storm patterns, rising wind, rain and sea levels, ocean acidification, and other climate change variables can contribute to spreading dead zones globally. Increased levels of carbon dioxide and warmer temperatures promote the growth of algal blooms. 

As sea temperature levels rise, the warmer waters hold less oxygen dissolved, leading to the easy formation of dead zones³. 

A dead zone can also occur seasonally as the mixing of the water column and other factors change. For example, dead zones start forming in the Gulf of Mexico during early spring and then spread during the fall as the water column mixes.  

Natural factors 

Human activity is a major cause of dead zones. However, a dead zone may also occur naturally due to some chemical, physical, and biological processes. For example, seasonal current upwelling can lead to hypoxic conditions in deep waters with excess nutrients and less oxygen. 

What are the impacts of dead zones? 

Hypoxic zones can affect the health of our oceans, the marine mammals and shore birds that live in and near them, and the people who depend on them for survival. Let’s glimpse at the environmental and economic impacts of an ocean dead zone: 

Environmental impacts 

As algae grows and dies, the decomposition process consumes oxygen, causing marine organisms to die. Typically, only a few organisms can swim out of the dead zone. Immobile species like oysters and mussels that need more oxygen to survive will slowly die due to low oxygen, displacing many fish species and disrupting marine food webs. 

Studies also link hypoxic zones in the ocean to reduced gonadal development, reduced sperm and egg quality, and low survival of fish larvae. 

Low oxygen levels in the oceans emit nitrogen, nitrous oxide, methane, C02, and other harmful greenhouse gases contributing to climate change. 

Economic impacts 

A dead zone doesn’t only affect wildlife but also impacts the economy.

Fishermen who live on these seas must venture farther from these low-oxygen areas to find fish. Consequently, the increase in travel distance escalates their transportation expenses. This journey may not be feasible for fishermen with smaller vessels.

Again, a dead zone can affect aquatic life, reducing the availability of commercial fish species like shrimps. 

For example, the dead zone in The Gulf of Mexico has increased the prices of larger brown shrimps as they are not commonly caught in a hypoxic zone compared to smaller shrimps. 

The hypoxic zone in our ocean also makes it unsafe for swimming. It impacts businesses in coastal areas, including hotels and restaurants.    

Dead zones in the world   

A hypoxic zone in salt and freshwater and the Great Lakes can occur worldwide. However, they are more common in coastal areas near watersheds. 

The number of dead zones can vary from year to year. Today, we have many hypoxic zones worldwide, which continue to increase. Here are some of the biggest dead zones in the world: 

• The largest dead zone in the world lies in the lower portions of the Black Sea. The dead zones occur naturally and form when water from the Black Sea mixes with the Mediterranean Sea. It covers almost 63,700 miles of the Gulf of Oman¹.
• The largest dead zone in the United States and the second largest dead zone in the world is in the Gulf of Mexico, covering around 5,364 square miles. This hypoxic Gulf Dead Zone occurs due to nutrient pollution from the Mississippi River Basin. Due to its increasing size, The Interagency Mississippi River and Gulf of Mexico Hypoxia Task Force was formed to reduce the dead zone to 1900 square miles by 2035. 
• The Chesapeake Bay on the United States's East Coast is one of the first dead zones scientists identified in the 1970s. Factories and urban centers on the bay's Western side release high nitrogen levels. Atmospheric nitrogen accounts for about a third of the nitrogen penetrating the bay. The Chesapeake Bay Foundation has since led many programs to curb pollution runoff and improve the bay’s water quality. The Chesapeake still has a dead zone, which can vary in size depending on the weather. 
• Lake Erie has a seasonal hypoxic zone extending to 10,000 square kilometers. The excess phosphorus from agricultural runoffs increases the growth of algal blooms, contributing to hypoxic conditions in Lake Erie. Indirectly, invasive species like zebra mussels also contribute to developing dead zones in Lake Erie.

How can dead zones recover or be prevented?                  

Over the years, dead zones have multiplied. But it isn’t all bad news. If proper actions are taken, some dead zones, especially those resulting from human activity like pollution, can be recovered. 

• One way to remove excess nutrients and preserve aquatic organisms is through the process of bioextraction. Bivalves like oysters and mussels can filter excessive nutrients from the water through bioextraction. Research from the NOAA and EPA reveals that cultivating bivalves through aquaculture can improve water quality and provide a sustainable source of seafood.   
• The US Environmental Protection Agency (EPA) outlines some best practices for reducing nitrogen and phosphorus nutrient levels varying from state to state. These strategies include reducing specific ingredients in fertilizers, implementing the best agricultural procedures to reduce pollution, etc. With animal agriculture contributing significantly to nutrient pollution, growing food directly for consumption instead of growing plants for animal feeds can substantially reduce the influx of nitrogen and phosphorus in our waterways. 
• On an individual level, you can contribute a few ways to help restore ocean dead zones. For example, reducing the use of fertilizers when growing vegetables, choosing to buy food with very little fertilizer application, maintaining a well-regulated septic tank system, and reducing the burning of wood and fossil fuels, amongst other actions 

Conclusion: What Is A Dead Zone?

Although dead zones may occur naturally, human activity, like nutrient loading during agricultural practices, has the most impact on forming dead zones. 

These dead zones signal global disaster for our environment. They are a financial threat to fishermen who rely on the ocean for their livelihood. In the face of a climate and economic crisis, taking immediate action has never been more critical.